conversation_module: update removed contact's conversation exactly once

When removing a contact, we need to update the ID of its linked
conversation (to be empty, which is the convention used to signify that
a contact doesn't have a linked conversation).

This used to be done via the updateConvForContact function, but there
were two issues (which are fixed by this patch):
1) if the user had multiple one-to-one conversations with the removed
   contact, then the function would be called multiple times (once per
   conversation) even though the update only needs to be performed once;
2) more importantly, if the contact's linked conversation ID didn't
   match any of its one-to-one conversations, then the update would not
   be performed at all. (Every contact should normally be linked to a
   valid conversation, but this can fail in practice due to
   synchronization issues.) As a result, the contact would remain linked
   to a conversation that no longer exists.

jami-client-qt#1673
jami-client-qt#1804

Change-Id: Ia75c5b27a4120bc48da5f449a874354326c6d96a